When exactly __init__() of bpy.types.Panel subclass must be executed?
The ____init____() function for BPY types (especially UI stuff) can be unpredictable and should not be used.  You likely want to be overloading the draw() method instead.

Example:
http://www.blender.org/documentation/blender_python_api_2_57_release/bpy.types.Panel.html